The present invention relates to a method of chemically thickening resins.
Various resins, such as ethylenically unsaturated resins, are utilized in a wide variety of end-use applications. For example, unsaturated polyester resins are particularly desirable as molded articles including bathtubs, water tanks, and automobile parts. In these instances, reinforcing fibers are typically blended with the resins to impart strength and lightweight dimensional stability to the article. Sheet molding compound ("SMC"), for example, is a mixture or combination which includes one or more resins, along with reinforcing fibers and inert fillers.
In particular, the automobile industry is increasingly using molded articles made from reinforced resins in place of heavier metal articles in order to provide more fuel efficient automobiles. Strength requirements in such applications are significant, and accordingly fibrous filaments are employed in high amounts, typically above 50 percent based on the weight of the article. Resin compositions containing such a high fiber amount, however, are often difficult to mold into articles with the reinforcing materials uniformly distributed.
More specifically, the production of molded articles with intricate shapes involves squeezing the reinforced resin material between male and female sections of a mold. Often during this process, a sizeable quantity of resin is forced out of the mold, typically exceeding 30 percent of the total weight. As a result of the sizeable resin loss, the formed article often becomes more highly filled in the region of the preform and is sparingly filled at the mold extremities or locations of maximum flowout.
To help enhance the physical properties of such formed articles, thickening materials have thus been employed. Conventional thickening materials have been in the form of metal oxides. Resins using the metal oxides, however, have been observed to display undesirable physical properties relating to brittleness and reduced moisture resistance.
As an alternative, isocyanate-containing resins have been proposed as chemical thickeners, such as described, for example, in U.S. Pat. No. 4,232,133. The `133 patent discloses the use of gels of ethylenically unsaturated monomer solutions which contain isocyanate resins. The resins are disclosed to be useful as SMC. The isocyanate resins are potentially advantageous relative to the metal oxides in that they exhibit improved stability and can be matured to form moldable compositions within a few hours.
Difficulties exist in using isocyanates as thickening resins. In particular, isocyanates in general are highly moisture sensitive. In such instances, side reactions often take place which produce urea and carbon dioxide. As a result, thickening tends to be non- uniform, thus adversely affecting thermal and hydrolytic stability of the resin, and resin physical properties.
In view of the above, there remains a need in the art for a method which allows for a resin to be chemically thickened while minimizing adverse effects to physical properties of the resulting article formed from the thickened resin.